In this work SAPO-34 zeolite membranes were hydrothermally synthesized on microporous ceramic α-alumina supports. These membranes were used to separate CO2 from CH4. The effects of synthesis parameters (synthesis temperature, synthesis time, coating repetition and silicon module in reaction mixture (nRM=SiO2/Al2O3)) on permeability and selectivity of synthesized membranes were investigated. Experiments were carried out at three levels of synthesis temperature: 185, 195 and 220 °C; synthesis time: 12, 24 and 48 h, coating layers: 1 and 2 and silicon module: 0.3, 0.4 and 0.6.
These membranes separated CO2 from CH4 with selectivity as high as 30.2 for a CO2 permeance of 3.8 × 10−8 mol/m2.s.Pa. The membranes are selective for CO2 because CO2 preferentially adsorbs, and it also diffuses faster than CH4.
The CO2/CH4 selectivity increased significantly as temperature increased from 185 to 195 °C. As trans-membrane pressure drop increased, the CO2 flux increased. The selectivity then remained approximately constant at further increasing synthesis temperature.
A reverse trend was observed for CO2 and CH4 permeances versus synthesis temperature. It reduced with increasing synthesis temperature from 185 to 220 °C.
The results indicated that the synthesis time had no significant effect on CO2/CH4 selectivity. An increase in synthesis temperature reduced selectivity slightly (less than 16 %). CO2 and CH4 permeances also enhanced up to maximum 30 %. This behavior was attributed to the dual effect of increasing synthesis temperature and time on membrane permeability and selectivity. Zeolite layers were thickened and larger crystals were formed at higher synthesis temperatures and times. Formation of larger crystals accelerated the rate of zeolite layer integration, which was responsible for gas separation, in one hand and reduced the density of deposited zeolite layer on the support, due to void increase, on the other hand.
Repetition of coating strongly increased CO2/CH4 selectivity and decreased permeances for both gasses.
The results showed that increasing silicon module had negative effect of selectivity. It was observed that an increase in silicon module from 0.3 to 0.6, slightly decreased selectivity.
The obtained permeances at this work were in the range of 7.6 × 10-9 to 4.7 × 10-7 (mol/m2 s Pa) for CO2 and 5.0 × 10-10 to 6.5 × 10-8 (mol/m2 s Pa) for CH4, respectively. The results showed that high permeable zeolite membranes were synthesized. In terms of maximizing the CO2/CH4 selectivity, maximum synthesis temperature (220 °C) with medium synthesis time (24 h) and two layer coating were selected. The silicon module in reaction mixture was equal to 0.3. Zeolite SAPO-34 membranes with moderate selectivities and permeances were successfully synthesized in this work. It found as good choices for the separation of CO2 from CH4 in commercial applications.
See more of this Group/Topical: Topical 8: Kister Distillation Symposium